This invention relates to lithographic printing plates and more particularly to planographic printing plates prepared from negative-working or positive-working photopolymerizable elements. Still more particularly, it relates to improved processes for preparing photopolymer lithographic printing plates using more convenient solvent developers and permitting latitude in the choice of supports.
Negative working lithographic printing plates having a photopolymerizable light-sensitive layer are well known in the art. Such plates generally comprise a hydrophilic support covered with a layer of a photopolymerizable hydrophobic composition. After imagewise exposure to actinic radiation which hardens the photopolymerizable layer imagewise, the unhardened portions of the layer are removed by washing with a suitable solvent, leaving an oleophilic polymer image on a hydrophilic support, suitable for use as a lithographic printing plate. Such plates are described, for example, in Alles U.S. Pat. No. 3,458,311, and Bauer et al. U.S. Pat. No. 3,751,259.
In British Pat. No. 1,294,896, there is described a method of making printing plates by leaching a photocurable composition from the unexposed areas of a porous support. A critical support porosity is required, and the hydrophilicity or oleophilicity of the support must be the opposite of that of the photocurable composition to provide an image composed of hydrophilic and oleophilic areas. This method and the other prior art require removal of the photosensitive material in the unexposed areas, whereas in the present invention, the original layer remains throughout the entire surface of the support and is subjected to leaching to produce hydrophilic and oleophilic areas in the layer.
Bauer et al. U.S. Pat. No. 3,751,259 discloses photopolymerizable compositions that are useful in the manufacture of presensitized printing plates for relief as well as planographic printing. There is no teaching that the binder component remains over the entire surface of the support and that in the unpolymerized image areas only the ethylenically unsaturated monomer has been substantially removed. This is substantiated by the fact that Bauer et al. require an additional step between development and application of suitable ink for printing.